I did exactly the same. I used 30mm square box tube but with 3mm thick steel so it will last the lifetime of the vehicle if not my own lifetime. I also used the plastic end caps after spraying the internal box tube with black hammerite foolowed by a good dose of waxoyl. The plastic end caps were sealed on with silicon sealant.
I, too, used aluminium strengtheners on the end caps but used angle section for added strength and alignment.
The bare metal side steps were cleaned, sanded and degreased and given a couple of coats of smooth hammerite and the underside and box tubes were then given a good coating of stone chip. I also sanded back the mounting plates on the chassis, coat of black smooth hammerite followed by a good coating of black stone chip once the steps were bolted in place.
I also mounted the steps tighter into the bodywork for a better ‘look’.
The only issue I found was that the rubber ‘covers’ had shrunk during storage (I had the steps off the vehicle and deconstructed for over a year) and are now about an inch too short in length with a slight gap in places to the end caps!